1. Field of the Invention
The present invention relates to lasers and particularly to a tunable solid state laser for producing a laser emission at any desired low or high laser gain transition over a preselected wavelength range.
2. Description of the Prior Art
It is desirable that a tunable solid state laser system generate a laser output at any low or high laser gain transition over the operating range of wavelengths of that laser system. However, such a tunable laser system often encounters higher gain transitions which prevent a desired low gain laser transition from oscillating.
Techniques, such as specially designed mirror coatings, birefringent plates, prisms and other loss elements at an undesired high gain transition, have been used to generate a laser output at a desired low gain transition. These techniques are cumbersome and expensive, and are not flexible when it is desired to tune from one wavelength to another.
Single or multiple stacks of birefringent plates, oriented at Brewster's angle to the laser beam propagation, are commonly used as tuning devices in laser resonators or cavities. The principle of operation is based on the fact that the amount of reflection loss suffered by an electro-magnetic wave at a dielectric surface is dependent on the polarization (i.e., p-polarized or s-polarized) and the angle of incidence of the incident wave, as well as the index of refraction of the dielectric medium (or material of a plate). At an angle of incidence equivalent to Brewster's angle, .beta., p-polarized light suffers no reflection loss, while s-polarized light suffers a reflective loss depending on parameters given above. Thus, the tuning operation of a single birefringent plate is based on the phase difference accumulated by components of a linearly polarized input beam polarized along the fast and slow axis of the birefringent element. In general, different amounts of phase retardation accumulated by the fast and slow wave components will result in an output beam which is elliptically polarized. An elliptically polarized output will suffer substantial reflection losses at all Brewster surfaces in the laser cavity. However, for those wavelengths for which the phase difference is an integer number N of 2.pi. radians, a linearly polarized input mode (p-polarized) undergoes a total retardation of N full waves, leaving the mode unaffected as it exits from the plate. This linearly polarized mode (p-polarized) suffers no reflection loss as it propagates past various Brewster surfaces inside the resonator, and thus corresponds to the operating eigenmode of the laser cavity.
In addition to the primary transmission maxima, the occurrence of secondary transmittance maxima must be considered in the design of a multiple plate birefringent tuning filter. Secondary transmittance maxima correspond to modes for which the total phase retardation of the thickness of the entire stack of one or more birefringent tuning filters corresponds to an integer number of half or full waves. Typically, the transmission value of secondary maxima may reach 75% or greater, depending upon the number of Brewster surfaces of the birefringent plates in the multiple plate birefringent tuning filter. Such high transmission values for these secondary peaks cannot be tolerated for broadband tuning operation of high gain laser systems. This is due to the fact that high secondary peak transmission values may allow strong transitions in the gain medium to oscillate, thereby limiting the tuning capacity of the birefringent tuner assembly. For flashpumped laser systems in particular, this wavelength switching can result in permanent damage to the laser rod end surfaces.
The above-discussed wavelength-switching problem is applicable to substantially all tunable solid state lasers which utilize various, associated, activator-ion-doped laser crystals to tune over various, associated wavelength ranges, whether each laser crystal is resonantly pumped or flashlamp pumped. The solution of such a problem in tunable solid state lasers is of particular interest and importance in relation to tunable lasers operating in the wavelength range between about 1.9 microns and about 2.1 microns, because of the many potential applications in medicine, optical communications and sensing. With the solution of this problem, it is believed that tunable solid state laser applications in other wavelength ranges, such as in the 1.4 micron to 1.6 micron eye-safe region, will also become more and more important.
At the present time, a number of scientists have attempted to solve this wavelength-switching problem, but without success.